There is, we can all agree, a yesterday, a today, and a tomorrow—so says, at least, our instinctive human intuition about life in the world. Many languages, and certainly English, are not satisfied, though, with this simple triple structure of time, and so the grammatical concept of tense can be at times confusing if not outright bewildering.
The term tense denotes the time of an action as it is marked by the form of a verb. A language may conceive of time in any way it wishes, with all manner of gradation and subtlety about just when something happened, is happening, or will happen. English depicts our temporal life with six standard tenses, but then turns a twist on that structure with the addition of what is called aspect. The English tense scheme is fairly simple to remember: just take that instinctive intuition of there being a yesterday, today, and tomorrow, and put those three distinctions under two categories, simple and perfect (the term perfect means completed). Thus, the six tenses in modern English are simple present, simple past, simple future; and present perfect, past perfect, future perfect. As to what is called aspect, there are basically only three: simple, progressive, and emphatic.
If I told you, for example, that the sun rose at 7:08 this morning, I would be using the simple past rose to ascertain a moment at which the event of the sun’s rising occurred, as marked by the appearance of the sun on the horizon. The scene I would be depicting is almost scientifically exact, and it concerns only the action of the sun’s rising, not that action together with any consequence it might have. To do that, I would have to employ the present perfect, the sun has risen, and my intentions in composing the sentence that way would have to be very different, perhaps to point outright to a specific result: the sun has risen, so now we can get to work. An action that is fully complete shows both the event and its consequence, and that is the function of the perfect tenses. By contrast, the simple tenses point only to the action, whether in the past, the present, or the future.
But how are we then to understand this statement: we walked along the beach as the sun was rising. The first verb, walked, is simple past, but the second verb, was rising, now takes on that curious twist we called aspect: the verb was is simple past, but it is used with the present participle of the verb walk. This combination produces what is called the progressive aspect of a verb, and it is used specifically to draw out an action (here in the past tense) in order to describe a broader scene: one thing happened (we walked) while another was underway (the sun was rising). Now it is true that the simple past alone will sometimes carry with it a sense of this same ongoingness (to walk at all happens across time), but the progressive aspect is used to stress the point. Indeed, we would probably have stressed the point too much had we used the progressive aspect in both verbs: we were walking along the beach as the sun was rising.
The lesson, then, is to consider very closely just what we see in our mind as we compose or revise a sentence—or, indeed, what we see in the mind of a writer whose sentences we are reading. We can leave it to the philosophers to decide whether time is merely a function of the human brain, or whether it is rooted more deeply in the mind or in nature itself. All of us, though, including those philosophers, live in the thick of things in the past, present, and future, and ordinary language presumes that those distinctions are real. The better we know the basic distinctions of time English grammar can make, the closer we are to reaching our goal of precision and style.